1. Field of the Invention
The present invention relates to lithographic apparatus and methods.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
For stepping or scanning of the patterning device or the substrate, a support structure for supporting the patterning device or the substrate may be coupled to a number of actuators. The actuators may control the support structure in six degrees of freedom, i.e. in three linear directions (x, y, z) and in three rotational directions (Rx, Ry, Rz). The actuators position and move the patterning device or the substrate in the six directions accurately with respect to each other and with respect to the projection beam in order to prevent irradiation errors, which errors may render the scanned substrate unusable.
The support system including the support structure and the number of actuators and its connections to other structures determines the dynamical properties of the system. For example, the support system may be characterized by mechanical eigen frequencies determined by the mass and inertia properties of the actuators and the support structure and their connections. A servo control system controlling the support system is designed by taking into account these dynamical properties.
Generally, flexible coupling devices such as leaf springs may be used between an actuator assembly including a first set of actuators for moving the support structure in a plane, for example in the x,y-plane, and a second set of actuators may be used for moving the support structure substantially perpendicularly with respect to said plane, e.g. x,y-plane, wherein each actuator of the second set is separately coupled to the support structure by flexible coupling devices, such as leaf springs. However, in such a configuration of the actuator assembly of the first set of actuators and the second set of separately coupled actuators, it is desirable to have a high horizontal stiffness of the attachment of the second set of actuators to the support structure to minimize cross talk to the horizontal direction. It may be difficult to design such an actuator coupling with a high horizontal stiffness and a vertical stiffness with desirable dynamical properties. Further, the dynamical properties may be predetermined during design of the support system such that the dynamical properties may be beneficial for the servo control of the support system, by enabling a high servo bandwidth that results in a high position accuracy.